It explored the origins of the Zodiac calendar and the mystical connections to it. Notably a first, the antagonist used precognitive abilities to discern extraordinarily accurate knowledge of Sarah's past within moments of meeting her, including things he should not know by normal means, such as her travels with the Doctor, something that she would only reveal to her closest friends. There was also a brief flashback to her reunion with the Tenth Doctor, his first appearance within the narratives of The Sarah Jane Adventures.

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Astrologer Martin Trueman causes a stir with his uncanny insights and predictions. Sarah doesn't believe in astrology, but she does believe that all is not as it seems with Trueman. Around her, people are falling under Trueman's sinister spell and even Mr Smith doesn't understand what's going on.

A young woman, Cheryl, is paying a regular visit to her astrologer, Martin Trueman. When she confides that she is spending her mortgage money to see him, Trueman breaks down and confesses that he is a con artist. As he stands by his window, unable to face her, he is hit by a shooting star. Suddenly, he's found a renewed belief in astrology.

Some time later, Rani and her mum have dragged Mr Chandra, Luke and Clyde to Martin Trueman's astrology seminar at East Acton New Theatre. Sarah has come, too, thinking she might do a story on Mr Trueman. Before entering the auditorium, each of them fills out a card with his or her birthday and gives it to Cheryl, who is now working as Mr Trueman's assistant. Luke, who was not really "born", does not have a birthdate, a fact that troubles him.

Mr Trueman begins his show by naming a random date and asking those born on that date to stand. Clyde stands, despite his stated intention not to; he is unnerved, but Sarah reassures him that it's a trick of persuasion. Mr Trueman selects another audience member. To the audience's astonishment, he divines her life story. Sarah can just convince her friends — and herself — that Trueman is just using cold reading techniques. Then Trueman selects her. He reveals an impossible amount of knowledge about seemingly ordinary Sarah's extraordinary travels and more importantly, about the Doctor.

Stunned and intrigued, Sarah stays after the show to talk to Trueman alone. She demands to know the source of his uncanny knowledge. He insists that astrology alone is responsible for his success. When he concludes the interview by warning Sarah against opposing him, she has no choice but to leave, as baffled and as unnerved as when she met him.

On the way home Clyde encounters Trueman, who his commanding power he used on him in the theatre and walked to his house and transfers some of the ancient lights power into Clyde to become his slave. In his trance Clyde suddenly seems to gain a new sense of purpose and direction, and sets off to meet Sarah and Luke back at 13 Bannerman Road... where he threatens to kill them, while Mr Smith stands idly by and insists that "nothing is happening". Clyde's hand glows with lethal energy as his eyes glow crimson red and he points it at Sarah, declaring that if she insists on interfering she must be destroyed.

Sarah and Luke talk Clyde out of threatening them, a feat Sarah attributes to Clyde's intrinsic goodness overriding any hypnotic techniques Trueman may have used.

The friends continue their discussion of Trueman. Mr Smith gives them background on astrology: a belief system widespread throughout the galaxy, he says and, in his opinion, mere superstition. It is a physical impossibility for "imaginary patterns in the sky" to affect reality, as the computer puts it. Luke theorises that, if that is the case, maybe the Ancient Lights Trueman speaks of are from another universe... the one that existed before Event One.

Mr Smith interrupts the theorising to patch through a television programme. Mr Trueman has taken over the airwaves and broadcast one of his astrology talks. This lecture/demonstration features a large, twelve-pointed star, each arm marked with a different sun sign. Trueman lights up the points of the star one by one, giving him control over people born under that sign.

He begins his those he considers brothers and sisters born under the sign of Gemini, a group that includes Rani's mum. The investigators return to the theatre to stop the broadcast, but the possessed Children of Gemini threaten to stop them. Clyde bluffs his way through by making up horoscopes.

The Ancient Lights create a portal to the theatre. Trueman reveals that Luke's theory on the Ancient Lights is true: in the previous universe, he claims, they controlled all lifeforms. The Ancient Lights survived the Big Bang and needed Martin Trueman to rule the world because he was the "Chosen One". Sarah tries to talk him out of it, but he will not listen.

Rani and Luke try to shut down the broadcast by flicking the mains switch off, but the Ancient Lights protect it with a force field. As they struggle to find a way through, Trueman's wheel reaches the sign of Aries, and Rani, an Aries, comes under Trueman's control.

Of the twelve sun signs on Trueman's wheel, only Taurus now remains. Sarah is a Taurus, but cannot think of what she and Luke can do with only seconds remaining before she, too, falls under Trueman's control.

Luke has an idea: since he was not born, he theorises, he doesn't have a star sign. He won't be under Trueman's control under any circumstances... so he is the only person able to destroy Trueman's wheel. Doing so severs the connection with the Ancient Lights and prevents their entry into the universe. Sarah pleads with Trueman to leave the theatre and escape with them, but Trueman refuses; he cannot bear to return to his former life and chooses to become "one with the stars" disintegrating into space much to Clyde's grief

As the friends celebrate back at Sarah's house, the clueless Earth authorities begin what the investigators know will be a fruitless search for Trueman. Luke and Sarah decide that, since Luke was able to use his lack of a birthday to save the universe, they will take today's date as his own equivalent of a birthday.

Not every person who worked on this adventure was credited. The absence of a credit for a position doesn't necessarily mean the job wasn't required. The information above is based solely on observations of the actual end credits of the episodes as broadcast, and does not relay information from IMDB or other sources.

The story was originally going to use the Mandragora Helix as a sequel to The Masque of Mandragora. However, it was decided that "Mandragora had to change too much and was too easily defeated in order to fit this plot", so a new foe was created instead. (DWMSE 23)

This is the third story in a row to involve a significant "mind control" theme.

Trueman telling Cheryl that she's "going on a long journey — a very long journey" in the pre-title sequence is an in-joke reference to Dimensions in Time, in which the Rani utters an almost identical line.

Near the end of Trueman's ritual, Taurus (♉) can be seen lit up on Trueman's zodiac wheel before he calls the sign, unlike what happens with other signs. Taurus itself was never activated on the wheel, and the power was cut before Trueman could announce it coming under the power of the Ancient Lights.